A riflescope (also called a “scope” for short) is an optical weapon sight based on the Keplerian telescope. In its most basic form, a riflescope is a Keplerian telescope with a reticle or cross hairs added to mark the “point of aim”. A riflescope must be mounted on a rifle such that its aiming axis (optical axis) is aligned with the rifle's barrel. However, due to mechanical tolerances, it is hardly possible to achieve a precise alignment the first time a riflescope is mounted on a rifle. The trajectory of the bullet must also be considered so additional corrections in the vertical direction might be necessary depending on target distance. In view of these considerations, riflescopes include means for precise vertical (elevation) and horizontal (windage) adjustments to the point of aim. The process of adjusting or correcting the point of aim of a riflescope so that the point of aim and point of impact coincide is called “zeroing-in”. Since a rifle's bullet follows a ballistic trajectory, a rifle is zerod-in at a known distance, say 100 m. At other distances, the rifle will shoot a predictable amount above or below the point of aim.
Early riflescopes were equipped with an external-adjustment system built into the mounts. Their point of aim was adjusted by mounts having micrometer windage and elevation mechanisms that moved the entire scope laterally and/or vertically. An advantage of external-adjustment scopes is that all the lens elements and the reticle remain centered on the same optical axis, providing highest image quality. The disadvantage of the external-adjustment mechanism is that the mounts must be able to support the entire weight of the riflescope under recoil. The external adjust mechanism is also bulky, heavy and susceptible to mud and dirt when used in the field.
Most modern riflescopes have an internal-adjustment mechanism using threaded screws mounted in turrets. The screws are connected to external knobs which are accessible by the shooter. Turning the knobs moves the reticle assembly inside the main tube against spring pressure. The knobs have clearly marked graduations around their circumference and many have a ball-detent system that clicks as the adjustment screws are turned. Each graduation or click represents a change in reticle position such that the point of aim is shifted by a small amount on the target. In modern riflescopes the graduations are commonly expressed as 1 cm at 100 m or 0.5 inch at 100 yards. The graduations may also be expressed in minutes of arc (MOA) or milliradians (mil). For the purpose of zeroing-in a rifle, 1 MOA is considered to be equal to 1 inch at 100 yards. Similarly, 0.1 mil corresponds to 1 cm at 100 m. These conventions are used in the present invention as well.
The opto-mechanical design of a riflescope with internal-adjustment mechanism is shown in FIG. 1. With reference to this figure, a riflescope is comprised of an objective lens 1 which forms a real image of the target at its focal plane. The image produced by the objective lens is upside down and laterally reversed. An image-erecting system comprising a pair of converging lenses 3a and 3b converts the image formed by the objective lens into an upright and laterally correct real image. An eyepiece lens 5 receives the image produced by the erector system and converts it into a magnified virtual image for the shooter to see.
FIG. 1 shows the reticle 20 being mounted coplanar with the objective focal plane. It is also possible to mount the reticle at the eyepiece focal plane. In either case, the shooter will see an image of the reticle superimposed on the image of the target.
The erector lenses 3a and 3b are mounted inside an inner tube 6 which is attached to the main riflescope housing 30 via a hinge 37. The inner tube 6 is able to tilt in both vertical and horizontal directions. The front section of the inner tube 6 is supported by the elevation knob 38 and the windage knob 39 against the pressure of a leaf spring 9 (see FIGS. 1 and 2). The purpose of this arrangement is to force the erector assembly to move together with the reticle so that the reticle always appears centered when viewed through the eyepiece. The reader is referred to U.S. Pat. No. 2,955,512 granted to Ernst Kollmorgen and John L Rawlings on Oct. 11, 1960 and U.S. Pat. No. 3,161,716 granted to D. J. Burris et al. on Dec. 15, 1964 for original disclosures of this mechanism.
Since its original invention more than 60 years ago, the mechanism described above has become the most widely used method for elevation and windage adjustment in riflescopes. Nevertheless, it has several fundamental limitations:                i. When the inner tube 6 is tilted to provide elevation or windage adjustment, the erector lenses 3a and 3b become decentered with respect to the objective optical axis. In telescope design, decentering the lenses is a fatal flaw: various optical abberations such as coma and astigmatism will be introduced and image sharpness is lost.        ii. The hinge 37 must provide two degrees of freedom and must be very precise. It is difficult and costly to make a precise hinge that can hold the rear end of the inner tube 6 at precisely the same position during elevation and windage adjustments or when the rifle recoils.        iii. The leaf spring 6 has very little room for compression and expansion. The spring pressure can change by temperature variations or other factors. The spring's point of contact with the inner tube 6 also shifts depending on the amount of elevation or windage adjustment applied (See FIG. 2).        iv. The bottom surface of elevation and windage knobs can touch (i.e. hold and support) the inner tube 6 only at a single point of contact once the inner tube is tilted. Furthermore, these points of contact shift around depending on the amount of elevation or windage adjustment applied (See FIG. 2).        v. The standard diameter for a riflescope's main tube is 1 inch in the USA and 30 mm in European countries that use the metric system. As a result, the gap between the inner tube 6 and the main riflescope housing 30 is only a couple of millimetres. This limits the range of horizontal or vertical shift that can be applied to the reticle (See FIG. 2) which in turn limits the maximum elevation adjustment that can be dialed in the riflescope.        
As far as the author knows, no remedy for weakness ‘i’ has been devised by the industry. As a result of weaknesses ‘ii’, ‘Hi’ and ‘iv’, elevation and windage adjustments in many of the riflescopes currently on the market are not repeatable. (In the hunting literature, this problem is known as “zero retention problem” or “tracking problem”.) To get around weakness ‘v’, some manufacturers have started manufacturing riflescopes with large-diameter main tubes (34 mm and even 36 mm). This practice has added significant bulk to their riflescope and has made mounting the riflescopes difficult as they no longer fit standard mounting bases and rings (which are either 1 inch or 30 mm).
In recent years, several optical adjustment mechanisms have been invented by the present author to replace the mechanical adjustment mechanism described above. U.S. Pat. No. 8,749,887 issued on Jun. 10, 2014 describes a riflescope wherein a pair of movable wedge prisms are positioned between the objective lens and its focal plane. U.S. Pat. No. 9,164,269 issued on Oct. 20, 2015 and U.S. Pat. No. 9,644,620 issued on May 9, 2017 describe mechanisms that utilize tiltable and rotatable wedge prisms for adjusting the point of aim in a riflescope.
The present invention discloses a simpler design: the point of aim is adjusted by attaching one or more wedge prisms in front of the objective. The prisms are fixed and the riflescope has no moving parts.